1. Field of the Invention
The present invention relates to an apparatus for the surveillance of an electronic security element in an interrogation zone. The apparatus has a transmitting device emitting at least one cyclic interrogation signal into the interrogation zone, with the interrogation signal causing the security element to deliver a characteristic signal, a receiving device having at least one receiver channel and receiving the characteristic signal, and a computing/control unit evaluating the signals received from the receiving device and producing an alarm when the presence of the security element is established.
2. Prior Art
To detect the presence of electromagnetic security elements in an interrogation zone, it is proposed in European Patent, EP 123 586 B to emit into the interrogation zone, in addition to two interrogation fields with the frequencies F1 and F2 in the kilohertz range, a field with a low frequency F3 in the hertz range. The two interrogation fields with the frequencies F1 and F2 cause a security element present in the interrogation zone to emit a characteristic signal with the intermodulation frequencies n.multidot.F1.+-.m.multidot.F2 (where n, m=0,1,2, . . . ). The low-frequency interrogation field causes the security element to be driven from saturation in one direction into saturation in the other direction at the clock rate of this particular field. As a result, the characteristic signal occurs cyclically at the frequency of the low-frequency field.
As an alternative solution, it has further become known to use only one interrogation field in the kilohertz range for excitation of the security element, with the characteristic signal of the security element occurring again at the clock rate of a low-frequency field cycling the soft magnetic, non-linear material between the two states of saturation.
Spikes, that is, high-frequency signals of a bandwidth greater than that of the characteristic signal of a security element, are produced by various interference sources. To name some examples, such spikes may be produced by motors, television monitors or passing streetcars. These spikes cause a lower detection performance and a higher risk of false alarm in an electronic surveillance system. Such false alarms are, of course, highly undesirable, since they are apt to confuse or irritate both staff and customers. To avoid a false alarm caused by spikes, it has only been known hitherto to provide the computing/control unit with predetermined spike templates. In practice, this means that while a spike having the shape of a known spike can be removed from the received signals, this does not apply to a spike differing in shape. This latter spike continues to be a problem in that it results in a poor detection rate and a false alarm of a surveillance system. The same applies in cases where the filtering of the receiving device is not accurately adjusted, or the inductivity or the Q factor of the receiving coils is outside of a predetermined tolerance.